Multiphoton absorption (“MPA”) is a nonlinear optical process that occurs through the simultaneous absorption of two or more photons via virtual states in an absorbing medium. Absorption of two photons is generally more common than absorption of more than two photons. For a given chromophore, the absorption processes take place at wavelengths much longer than the cut-off wavelength of its linear (single-photon) absorption. In the case of two-photon absorption (2PA), two quanta of photons may be absorbed from a single light source (referred to as degenerate 2PA or direct 2PA) or from two sources of differing wavelengths (referenced as non-degenerate 2PA). In practice, the former 2PA phenomenon is much more pervasive in two-photon applications. Thus, in considering the practical exploitation of a direct 2PA processes, it is important to recognize the following useful features of the 2PA phenomenon, which are based on the fact that 2PA scales nonlinearly with the squared intensity of the incident laser beam. Firstly, incident light at a low frequency (energy) may be upconverted to an output, emission light at higher frequency. For instance, near infrared (“NIR”) may be upconverted to ultraviolet (“UV”) via a 2PA upconversion process. The deeper penetration of incident NIR light than UV light that may be hazardous with prolonged exposure. Highly localized excitation via a 2PA process, as compared with one-photon processes, allows precise spatial control of in situ photochemical or photophysical events in the absorbing medium, thereby minimizing undesirable activities such as photodegradation or photobleaching. Finally, fluorescence (i.e., light emission via relaxation from singlet excited state to ground state) when properly manipulated, allow for information/signal feedback or amplification in conjunction with other possible, built-in effects, such as surface plasmonic enhancement effect.
It is anticipated that ingenious utilization of these basic characteristics of 2PA process will lead to practical applications beyond bio-medical fluorescence imaging, data storage, directed energy protection, hazardous chemical detection, microfabrication of microelectromechanical systems (“MEMS”), photodynamic therapy, etc. In the past two decades, significant advances have been made in the fundamental understanding of structure-property relationship of 2PA processes, which have led to the design and synthesis of two-photon absorbers having large cross-section values. While theoretical studies have suggested further enhancement of 2PA cross-section is still possible for certain applications, the two-photon-property requirement has essentially been met by state-of-art chromophores. Because of the possible property-processing/fabrication trade-off, optimization of secondary properties, e.g., thermal and mechanical properties, which are important in processing materials into various, useful forms (films, coatings, fibers, windows, etc.) and configurations, should not be neglected. Furthermore, device fabrication and integration conditions should be considered in parallel with the continuing efforts to enhance the effective cross-section (a) values. For the aforementioned solid forms, polymers can offer many advantages such as the flexibility in fine-tuning the material properties, and the availability of many processing options.
Aromatic-heterocyclic rigid-rod polybenzobisazoles (“PBXs”), such as polybenzobisthiazole (“PBZT”), polybenzobisoxazole (“PBO”), and polybenzobisimidazole (“PBI”), are well known for their superior thermal and mechanical properties and their promising nonlinear-optical (“NLO”) and opto-electronic properties. These stiff-chain and fully conjugated polymers are typically synthesized in acidic media, such as polyphosphoric acid or methanesulfonic acid/phosphorus pentoxide mixtures. However, PBXs have the disadvantage that their practical solubility, which is limited to only strong acids. It is known to practitioners in the field that PBXs without suitable solubilizing pendants are practically insoluble, even at low molecular weights, in organic solvents commonly used in solution-based polymer processing. For structurally unmodified PBXs, such a drawback has constituted a serious problem in film fabrication and coating applications for NLO devices.
In U.S. Pat. No. 6,730,793, Ramamurthi Kannan et al., issued May 5, 2004, a suite of quadrupolar two-photon absorbing compounds based on a “Donor-Acceptor-Donor (D-A-D)” structural motif was described. Among these 2PA compounds were examples based on diphenylamine-benzobisthiazole-diphenylamine motif, which have high two-photon absorptivity. Traditionally, the formation of a benzene-fused bis(heterocycle), benzobisimidazole, benzobisoxazole or benzobisthiazole proceeded via a tandem condensation/dehydration process of a tetrafunctional monomer, i.e., 1,2,4,5-tetraaminobenzene, 4,6-diaminoresorcinol, or 2,5-diaminobenzene-1,3-thiol, respectively. A difunctional monomer, such as an aromatic dicarboxylic acid or acid chloride, drives the polymer-forming process. Viably, an alternative polymerization process entails a monomer comprising a preformed benzobis(heterocycle) structure with suitable reactive endgroups can engage in the formation of aryl-aryl bonds (as polymer-forming reaction) with a suitable co-monomer.